The ultrastructure of chilling stress

Chilling injury to crop plants was first described 70 years ago and has bee n systematically investigated with electron microscopy since the late 1960s . Chloroplasts are the first and most severely impacted organelle. Thylakoi ds swell and distort, starch granules disappear, and a peripheral reticulum (vesicles arising from inner membrane of chloroplast envelope) appears. Ch loroplast disintegration follows prolonged chilling. Mitochondria, nuclei a nd other organelles are less susceptible to chilling injury. Organellar dev elopment and ontogeny may also be disrupted. The inherent chilling sensitiv ity of a plant, as well as the ability of some species to acclimate to chil ling, influence the timing and appearance of ultrastructural injury with th e resulting outcome being mild, moderate, or severe. Other environmental fa ctors that exacerbate injury are irradiance, chilling duration, and water s tatus. The physiological basis for chloroplast swelling may be linked to ch illing-stable starch-degrading enzymes that produce soluble sugars thus low ering stromal water potential at a time when chloroplast photosynthate expo rt is reduced. Thylakoid dilation appears to be related to photo-oxidative conditions produced during chilling in the light. The peripheral reticulum is proposed to increase surface area of the transport-limiting membrane (ch loroplast inner membrane) in response to the chilling-induced reduction in metabolite transport. Many of the ultrastructural symptoms appearing during moderate stress resemble those seen in programmed cell death. Future resea rch directions are discussed.